He was born at Forstena manor in Västergötland. His parents were Märta Nilsdotter Posse and Torsten Lennartsson, of the noble house Forstena, who was supporter of King Sigismund and, for a while, the commandant of Älvsborg fortress. Young Lennart's parents fled to exile in the year of his birth because his father had confessed to being loyal to the deposed Sigismund. Lennart was taken care of by relatives - his father returned to Sweden only when Lennart was around twenty. His paternal uncle Anders Lennartsson was Lord High Constable of Sweden and trusted by Duke Charles, but he fell at the Battle of Kircholm in 1605.