One day, Jenny Holzer saw a picture. It showed a woman with her legs spread, lying in a forest after being raped and murdered. The drawing, by the unflinching German artist Käthe Kollwitz, provoked Holzer to write some text in the voice of an imagined adviser to the attacker. "Crack the pelvis so she lies right. This is a mistake. When she dies you cannot repeat the act. The bones will not grow together again and the personality will not come back."  Another text, for the same Under a Rock series, read: "Nothing will stop you the mad foreigner. No one sees you walking with a tick tick in your dick. You set a bomb in the Queen City of the big nation to start the war that stops when silence breaks the ears of dead people." She made these words flash across LED signs and carved them into stone benches. One of the LEDs is at the Sprüth Magers gallery in London for a show called Sophisticated Devices, featuring Holzer's work from the 1980s, including her collaboration with New York graffiti artist Lady Pink.  In one work, a Holzer slogan howls from Lady Pink's graffiti through the window to the street beyond: "I am not free because I can be exploded anytime." It's characteristic Holzer: text as punchy as a headline, yet confusing to unsuspecting passersby. Inside, there are marble benches inscribed with texts. Visitors seem unsure if they're permitted to sit on them. Holzer hopes they will.  Meanwhile, at Sprüth Magers's sister gallery in Berlin, Holzer has a show of more recent work called Endgame. For this, she plundered declassified US war documents, including autopsy reports, FBI emails from Guantánamo and letters from detainees – and meticulously reworked them in paint.  Although two decades separate the work in the two shows, they are surely the work of a politically conscious artist. "On the worst days, I don't feel like an artist," laughs Holzer when I put this to her. "The continuity between this show and the other is tough content. That's what I gravitate towards." Where does this dark aesthetic come from? "The desperate things seem to require attention, the lovely things seem to elicit celebration. If I had to choose, I would go to the awful in the hope that doing something could yield a happier result."  Holzer, now 61 and living in Hoosick Falls in upstate New York, is perhaps best known for the slogans she put on everything from T-shirts and caps to LEDs and even condoms in New York in the late 70s and early 80s. In those days, she would skulk around at night putting up posters with texts culled from Karl Marx, Susan Sontag and other intellectuals: "The desire to reproduce is a death wish" was one; "Romantic love was invented to manipulate women" another.  "I would sneak around the morning after I'd pasted them up to see if anybody would stop," recalls Holzer. "That's the test of street art – to see if anybody stopped. People would cross out ones they didn't like and would star others. I liked that people would engage with them."  Later, she put up a huge electronic billboard above Times Square that read: "Protect me from what I want." There's an endlessly reproduced shot from 1983 of Lady Pink, cigarette in hand, walking down a New York street wearing denim shorts and a singlet inscribed with the Holzer text: "Abuse of power comes as no surprise." Have you ever worn one of your T-shirts? "No, that would be mortifying. Shoot me if you ever see me in one."  Holzer didn't set out to be a conceptual artist who uses text, but rather an abstract painter. "I wanted to be soft like Rothko and ruthless like Ad Reinhardt." So you were inspired by men not women? "There was a real paucity of role models. When I was a kid, the only artist I knew about was Picasso courtesy of Life magazine. And, as a little female kid in Ohio, it was hard to identify with Picasso: his life with his babes and my life with my cat were rather different."  We're chatting amid her flashing LED installations in the gallery's basement. Holzer is nursing a broken arm, the result of being smashed into by a cyclist in a park in Paris three weeks ago. Did the cyclist stop? "No, but a nice granny and grandpa hovered over me. Now I've got all kinds of metal and garbage in my arm. I'm the most scanned individual at an airport because I'm full of metal." If only she'd been wearing a T-shirt with the slogan: "Cyclists should not race through parks like sociopathic boneheads."  Holzer gave up her dream of emulating Rothko and Reinhardt in 1976 when, after a liberal arts education followed by art school, she took part in a study programme at the Whitney Museum in New York. Holzer's tutor Ron Clark smothered her and her fellow students in texts. "He gave us a wonderful yet absolutely daunting reading list that, happily, I reacted against. I reduced all the reading to one-liners." Her version of the list, turned into a work called Truisms, features such nuggets as: "A lot of professionals are crackpots."  Why put these words on posters? "I knew the reading list had genuinely important material and I knew most people in the world wouldn't read it. So I thought, 'Maybe I can convey some of the valuable content in an accessible way.'"  In the decades that followed, Holzer developed more sophisticated ways of using text; her work moved from the streets into venerated art institutions, becoming infinitely more distinguished visually. As an example of her painterly sensibility expressing itself through electronic light, she is most proud of her 2001 installation for Berlin's Neue Nationalgalerie: amber text scrolled along 13 LEDs 49 metres long that hung below a ceiling of black-painted steel beams, all of this inside Mies van der Rohe's building. "We managed to turn the air amber – that was my equivalent to the halo in a Rothko."  This installation marked a break in Holzer's work. From that point on, she would use only other people's words. "Because I'm a bad writer. I was so relieved to stop, not just for sheer laziness but because I was able to expand what I could do when I work with other people." One such project saw her projecting In Praise of Feeling Bad About Yourself, by Nobel-winning Polish poet Wisława Szymborska, on to the Chicago Tribune Tower. She remembers fondly what the poet – who died aged 88 in February – thought of Holzer's projections. "She watched them scroll over a castle and a river and laughed. She was a good laugher."  Holzer also made entire novels scroll across a convention centre in Pittsburgh. "Computers have big brains, so you can put a lot of stuff up. People who worked there would see something different every day." Holzer now often projects words on to waves or around the contours of buildings. Why? "This is maybe ridiculously pretentious, but I want it to be a little like song. You know, rise and fall."  Any suggestion that Holzer has softened politically is confounded by her work since 2004, much of it featured in Endgame. Catalysed by her loathing for war in Iraq and Afghanistan, she studied redacted, declassified documents from the US's National Security Archive and made silkscreen paintings of the most striking. They include maps used to plan the invasion of Iraq; these look like parodies of US foreign policy but, disturbingly, are the real thing. Big arrows are marked with such words as "Exploit" and "Isolate". She laughs: "They really did say shock and awe!" In 2010, she started hand-painting copies of some of these documents – why the return to painting after a three-decade layoff? "I wanted to show time and care. I wanted it to be an indicator of sincerity and attention. I wanted it to be human."  Does the work reflect your own political views? "Don't be a psycho killer – that has to be a goal, right?" she says, laughing again. "Having torture seemingly normalised is, I don't think, a positive thing. Not enough people have stated that." But, crucially, Holzer doesn't overtly state that: she leaves us to arrive at that belief through looking at her work. "I think the material speaks for itself," she says. "There's no reason for me to give my pathetic opinion."
Jenny Holzer's stark, often shocking slogans have graced T-shirts, posters, LEDs and even condoms. As a show of her early work opens, the US artist tells Stuart Jeffries why she's drawn to dreadful things