Silas trained under Victor MacAuley. Silas competed in the 1928 Olympics and beat Clarence DeMar, five time winner of the Boston Marathon. Two years later Silas won the 1930 Halifax Marathon. In the same year, Silas McLellan finished sixth in the 1930 British Empire Games marathon. McLellan also finished 9th in the Boston Marathon 19 Apr 1930.
He was inducted into the Nova Scotia Sport Hall of Fame. McLellan died at age 76 in Noel, Nova Scotia.