In 1960 he returned to Australia to play a sheep drover in The Sundowners, starring Robert Mitchum and Deborah Kerr.
He appeared in seventy-two films between 1956 and 1998, the most notable of which include The Sundowners (1960), In Search of the Castaways (1962), The Sons of Katie Elder (1965), Major Dundee (1965) and Logan's Run (1976, which his father directed).
Lennart's later screen credits include A Life of Her Own, Love Me or Leave Me, Merry Andrew, The Inn of the Sixth Happiness, Please Don't Eat the Daisies, The Sundowners, and Two for the Seesaw.