[2] He and his seven brothers were educated at Christ's College, Christchurch, and five of them played first-class cricket in New Zealand.

He captained Christ's College at cricket and rugby,[4] and was the top New Zealand school student of his year, excelling widely, especially in languages.

[6] In 1879 he joined Thomas Smith Duncan, who had set up his law firm in Lyttelton in 1857.

[4][7] As of 2020, Duncan Cotterill has offices in Auckland, Wellington, Nelson and Christchurch, and is the largest law firm in the South Island.

[8] Cotterill served on the board of governors of Christ's College from 1896 to 1943,[4] and was also president of the Canterbury Law Society.