Surname L
| SURNAME | TOWNSHIP | RESEARCHER |
| Ladd | Wauconda | Chris Ladd |
| Lagerstrom | 1878-1967 | Eric Robert Lagerstrom |
| Lambrecht | Sandy Demlow | |
| Lamphere | Laura L. Riley | |
| Lasco | Antioch | Dawn (Hanke) Knauft |
| Laube | Marilynn Wienke | |
| Lawrence | Louella | |
| Leber | Waukegan | Joyce Higgins |
| Leihy & var. | Fred and Margie Leihy | |
| Leikam | Verdell Leikam | |
| Leistikow | Antioch | Dawn (Hanke) Knauft |
| Lennington | John Winters | |
| Lesner⁄Leszczynski | Laura L. Riley | |
| Lewis | Patti Fellows | |
| Liese⁄Lies | Vernon Township | Peter Brandl |
| Lohman | Barrington & Ela - 1840s | Diane Strothers |
| Lubkeman⁄Luebkemann | Antioch & Mundelein | Denise Parsons |
| Ludlow | Brenda | |
| Ludwig | Highland Park | MMVindy1 |
| Lunham | West Lake Forest (West Deerfield then?) 1880-1925? | Charlotte Strasser |
| Lux⁄Laux | Warren | Don Finel |
| Lynn | Janice Lynn |