Another source states:
In all, the House of Representatives has impeached only 19 federal
officials, and the Senate has conducted formal impeachment trials with
seven acquittals, eight convictions, three dismissals and one
resignation (Nixon’s) with no further action.
This is inaccurate as Nixon was never impeached in the first place. He resigned while the House was considering impeaching him.
Two Presidents (Johnson and Clinton) were impeached but acquitted.
Only two people other than a judge or a President were impeached.
Secretary of War William W. Belknap was impeached for graft and
corruption, but acquitted after resigning on August 1, 1876.
U.S. Senator from Tennessee William Blount who was impeached for
conspiring to assist Britain in capturing Spanish territory. In
Blount's case, the Senate refused to accept impeachment of a Senator
by the House of Representatives, instead expelling him from the
Senate on its own authority.
Expelling vs Impeachment
Art. I, § 5, Clause 2 of the U.S. Constitution states: "Each House may determine the rules of its proceedings, punish its members for disorderly behavior, and, with the concurrence of two thirds, expel a member.", which is how the Senator was expelled.
In contrast, impeachment is provided for in Art. II, § 4: "The President, Vice President and all civil officers of the United States, shall be removed from office on impeachment for, and conviction of, treason, bribery, or other high crimes and misdemeanors." The impeachment process is in Art. I, §§ 3 (House impeachment) and 4 (Senate trial) and referred to in Art. III, § 2 (constitutional venue rules don't apply to impeachments).
The other fifteen impeachments were all of judges and eight of those judges were convicted by the Senate (incidentally, seven of those eight judges were exclusively trial court judges as opposed to appellate judges).
Four of them were acquitted following a trial in the Senate (one was a U.S. Supreme Court Justice acquitted on charges of being politically driven, two were trial court judges charged with abuse of power and one was a trial court judge charged with corruption).
Three of the judges, all serving as trial court judges, resigned following their impeachment resulting in the dismissal of the cases before the Senate reached a verdict (one was charged with abuse of power, one with rape and obstruction of justice, and one with drunkenness).
This source also notes that:
And just like at the federal level, impeachment at the state level is
extremely rare. For example, the state of Illinois has impeached only
two officials in its entire history—a judge in 1832-33 and a governor
(Rod Blagojevich) in 2008-09.
Thus, there have been eight people removed involuntarily from federal office via the impeachment process since the United States Constitution took effect in 1789. Wikipedia confirms the 19 impeachments and their resolutions discussing each one in turn. The eight removals were as follows:
Judge John Pickering (District of New Hampshire) for drunkenness and unlawful rulings. Removed on March 12, 1804.
Judge West Hughes Humphreys (Eastern, Middle, and Western Districts of Tennessee) for supporting the Confederacy. Removed and disqualified on June 26, 1862. (He actually served simultaneously as a Union judge and a Confederate judge.)
Associate Justice Robert Wodrow Archbald (United States Commerce Court) and Judge (Third Circuit Court of Appeals) for improper acceptance of gifts from litigants and attorneys. Removed and disqualified on January 13, 1913.
Judge Halsted L. Ritter (Southern District of Florida) for champerty, corruption, tax evasion, and practicing law while a judge. Removed on April 17, 1936. (Champerty means financing or purchasing someone else's lawsuit.)
Judge Harry E. Claiborne (District of Nevada) for tax evasion. Removed on October 9, 1986.
Judge Alcee Hastings (Southern District of Florida) for accepting a bribe, and committing perjury during the resulting investigation. Removed on October 20, 1989.
Chief Judge Walter Nixon (Southern District of Mississippi) for perjury. Removed on November 3, 1989.
Judge Thomas Porteous (Eastern District of Louisiana) for making false financial disclosures. Removed and disqualified on December 8, 2010.