Rennie attended schools at Hamilton then Toronto where he received his first medical degrees at Toronto's Trinity Medical College. He proceeded to further studies in London where he received the Royal College of Physicians licentiate (LRCP) diploma. He received his LRCS licence at Edinburgh. Rennie studied further at Paris and Vienna.[1][2]
He became the medical director of the Commonwealth Life Insurance Company and surgeon-in-chief of the Toronto, Hamilton and Buffalo Railway. He was also appointed Hamilton's chief coroner.[1]