Alexander Slidell MacKenzie (24 January 1842 – 13 June 1867) was an officer in the United States Navy during the Civil War.

Born in New York, MacKenzie was appointed midshipman 29 September 1855. Serving in Hartford on the China station at the outbreak of the Civil War, he returned to the United States and joined Kineo , in which he served during Farragut’s daring dash past Forts Jackson and St. Philip on the lower Mississippi to capture New Orleans in 1862. During 1863 and 1864 he participated in the blockade of Charleston and the attacks on Fort Sumter and Morris Island. After the end of the war, he returned to the Far East in Hartford, in which he served until 1867, when he was killed on Formosa while leading a reprisal attack against those responsible for the deaths of the entire crew of the American merchant bark Rover.

Three ships of the Navy have been named USS MacKenzie in his honor.