The family business, which is engaged in construction, property development, quarrying, and the manufacture of timber house kits, windows and doors, has switched to the Spanish bank from Glasgow-based Clydesdale Bank.
Asked why M&K MacLeod had decided to move bank, company secretary Jane MacLeod replied: "We just wanted to go for growth and Clydesdale Bank are less enthusiastic about development these days.
"Santander have been more than welcoming. We are very impressed by their enthusiasm and the fact they are very keen to take on...businesses like ours."
M&K MacLeod, which counts local authorities and housing associations among its major customers and also operates a petrol station in its home town of Lochgilphead, has about 200 staff. It also provides work for about 200 sub-contractors.
Its latest published accounts show it achieved a rise in pre-tax profits to £753,732 in the year to May 31, 2012, from £675,189 in the prior 12 months, as turnover rose from £19.2m to £22m.
Ms MacLeod signalled accounts for the year to May 2013 would show further rises in turnover and profits.
Asked about M&K MacLeod's success in recent years, amid tough times for the construction sector, Ms MacLeod said: "We have had to diversify. We are doing things like biomass, and more affordable housing than private developments. We are just versatile, moving where the market is."
M&K MacLeod was set up by twin brothers Murdo and Kenny in 1975, and continues to be owned by them. Current projects include construction of the Atlantic Islands Centre on Luing, the building of 32 housing units in Campbeltown for Argyll Community Housing Association, and the installation of biomass systems for schools on Islay.