He was said to be particularly proficient in applied mathematics, a skillful topographical draftsman, a lucid lecturer, and a man of high professional ideals. His students found him a strict and, at times, severe teacher, but many discovered the kindness behind his somewhat dour exterior, particularly when they returned to him after graduation for help and advice.
As Borough Engineer, McMillan designed Princeton's first sewerage system and planned the town's modern streets. He was often consulted by the College, the Princeton Water Company, and the state of New Jersey about sanitation and water problems.
He retired in 1914, having served thirty-nine years as Professor of Civil Engineering and Applied Mathematics, and lived to see his pioneering efforts culminate in the formation of the School of Engineering in 1922.