Of Dalmatian descent, Mrkusich was born in Dargaville and moved to Auckland in 1927. After completing a commercial apprenticeship in Writing and Pictorial Arts in 1942 he attended Seddon Memorial Technical College to take life drawing classes and later became a partner in Brenner Associates in 1949, working as a colour consultant and architectural designer as well as offering exhibition and display design. Following the close of the firm in 1958, Mrkusich received various architectural commissions such as murals, stained-glass window and mosaic projects. He also embarked upon a fulltime painting practice. Heavily influenced by developments in international abstract art, Milan was the earliest painter to pioneer abstract modernism in New Zealand, seeking ‘truths’ of colour and form. Among his commissions are the Church of the Holy Cross and the CSR Building in Auckland and Te Papa, Wellington.