Lammert Jan Overmars was born in 1863 in Olst, Overijssel where both his parents were farm workers. When Lammert was two, his mother Antje Banté died aged 39. In 1866 his father Mannes remarried to Derksken Stoelhorst (1826-1904), who had recently been widowed.
On 5 April 1888 in Deventer, Lammert married Reintje de Winter (1864-1926). Reintje was the daughter of Gerrit Jan de Winter (1833-1903), a farmer and clogmaker who had recently built a public house, and his wife Dina Kluitenberg (1837-1892). Lammert and Reintje had three surviving children, a daughter who died in infancy and a stillborn son.
Lammert worked as a pork butcher until shortly before his death, when he began work as a warehouseman. Lammert died on 23 June 1897 in Deventer aged 34. He left behind Reintje and three children. After his death, Reintje moved to Haarlem where she married Lammert's brother-in-law Christiaan Hietkamp (1860-1918). They had further children and later lived in Zutphen, Gelderland.
- 1888: Lammert Jan Overmars, 25, spekslager
- 1897: Lammert Jan Overmars, 34, pakhuisknecht
Father: Mannes Overmars, day labourer, living in Olst
Mother: Antje Banté, deceased
Bride: Reintje de Winter, 23, no occupation, living in Deventer
Father: Gerrit Jan de Winter, clog maker, living in Welsum, Olst
Mother: Dina Kluitenberg, no occupation, living in Welsum, Olst
Witnesses: Hendrikus Albers, 47, groomsman; Lammert Overmars, 42, postman, uncle of Lammert; Johannes Eikenaar, 28, day labourer, brother-in-law of Lammert and Albert Pessink, 29, day labourer, brother-in-law of Reintje.