Malachy Costello studied painting at the Crawford College of Art, Cork and the Institute of Technology, Sligo. Since graduating in 1983 he has held numerous solo exhibitions and is a regular contributor to numerous group exhibitions;  the Irish Exhibition of Living Art in 1984, the RHA Annual, Dublin, the RUA, Belfast, Oireachtais Exhibition, Eigse, and many others over the years.

His paintings are in public collections of the OPW, Bank of America, Co. Councils of counties Sligo and Roscommon, and other corporate bodies, as well as many private collections in Ireland and abroad.