[5] After receiving his education at a local school, he attended Marischal College in Aberdeen[3] but despite his father's reservations, he sought to follow him in an agricultural career.
[6] Initially, McCombie's employment was within the extensive family farming business, part of which was transporting cattle to the borders of Scotland and into England for fattening.
[6] During the 1820s[a] he rented the arable 1,200-acre (4.9 km2) Tillyfour Farm from his father[3][6] and began the process of building up his own herd of black polled cattle.
[3] When rinderpest struck northeast cattle herds in 1865, McCombie organised Aberdeenshire farmers to raise a compensation fund that made possible the rigorous slaughter policy that extirpated the disease, thus providing the model for modern disease control measures among farm stock.
[3] He became the first tenant farmer elected to a Scottish constituency in 1868 when he represented the Liberal Party as the western division of Aberdeen Member of Parliament.