Born in Dundee, Mills moved first to Paisley then to Bridgeton, Glasgow as a young boy.
In 1909, he began playing in the Scottish Junior Football Association with Shettleston F.C., Vale of Clyde F.C.
In 1923, he moved to the United States, but does not show up on a professional roster until the 1924–1925 season when he played twenty-two games for the Philadelphia Field Club of the American Soccer League.
Over the next sixty years, Mills forged a legendary coaching reputation as he led multiple teams at the collegiate, amateur, professional and national levels.
He spent only one season at Penn before becoming the head coach of the boys' team at The Episcopal Academy.
purchased the professional Philadelphia Nationals of the American Soccer League and elevated Mills to head coach.
Beginning in 1948, Mills' led the Nationals to a string of league (1949, 1950, 1951 and 1953 ) and Lewis Cup (1949, 1951 and 1952) titles.
His year at Penn and his work at the Episcopal Academy brought him to the attention of Haverford College which hired him in 1949.