In 1853–1855, as a passed midshipman, Cummings was assigned to the steamer Fulton. Simultaneously promoted to the ranks of master and lieutenant in September 1855, he next served in the sloop-of-war Dale in the African Squadron.
In 1859 Lieutenant Cummings was attached to the steam sloop-of-war Richmond, in which he served during a 1860–1861 Mediterranean deployment. Then, with Cummings as first lieutenant and executive officer, Richmond was deployed to the Gulf of Mexico. In April 1962, she participated in the bombardment of Forts Jackson and St. Philip in the Mississippi River delta, in the capture of New Orleans and in subsequent operations up the Mississippi River. On 5 August, Cummings was promoted to lieutenant commander.
On 14 March 1863, Lieutenant Commander Cummings was mortally wounded as Richmond attempted to pass the batteries at Port Hudson, Mississippi. He died on the 18th while en route to New Orleans, and was interred in his family's plot at Laurel Hill Cemetery, Philadelphia, Pennsylvania.