Born in Montreal, Quebec, Lanthier was educated at McGill University. In 1960, he joined the accounting firm of Peat, Marwick, Mitchell & Co. in Montreal rising to become partner in 1972. He worked as a Managing Partner in the London, Ontario office from 1977 to 1982, in the Ottawa office from 1982 to 1984, and the Toronto office from 1984 to 1989. From 1989 to 1993, he was Vice-Chairman of the Greater Toronto Area. From 1993 to 1999, he was Chairman and CEO of KPMG Peat Marwick Thorne (renamed to KPMG in 1996).
In 2001, he was appointed to the Board of Directors of the Bank of Canada. He has also served on the Board of Directors for Ellis-Don Inc., Emergis Inc., Gerdau AmeriSteel Inc., RONA Inc., Torstar Corporation, TMX Group Inc., Canada Life Assurance Company,Biovail Inc and Zarlink Semiconductor Inc.
In 1999, he was made a Member of the Order of Canada. In 1982, he was awarded F.C.A. by the Ontario Institute of Chartered Accountants.He received the Institute's award of Outstanding Merit in 2002.