City of St. Louis James McIlvaine "Mac" Riley (May 16, 1849 – May 6, 1911) was an American civil engineer and fraternity founder.

He worked for the United States Army Corps of Engineers on the Mississippi River and was a surveyor with the City of St. Louis.

[3] On December 30, 1852, Riley's mother married Clinton Odell Dutcher, a commission merchant.

[1] His cousin, Marshall McDonald, was an adjunct professor of chemistry, mineralogy and geology at VMI.

While at VMI, James Frank Hopkins, Greenfield Quarles and Riley became close friends and founded Sigma Nu fraternity on January 1, 1869.

[3] In 1878, Riley began working as a civilian with the United States Army Corps of Engineers.

[3] Riley was elected as the first regent or national president of Sigma Nu fraternity in 1870; he was reelected to a second five-year term in 1875.

[3] Before the 1902 grand chapter meeting in Indianapolis, he wrote, "I will be with you do I not have nervous prostration; in other words: I live very quietly and so am fearsome of results; not being accustomed to attend functions of any kind.