In 1935, Washington began to issue license plates by county. A one- or two-letter prefix denoted the county, and letters were assigned by the order of population, with King County, the most populous, receiving letter A, followed by Pierce with B, and so on. After letter Z, the 13 smallest counties received two-letter prefixes derived from the county name.
In 1954 the system remained the same, except the county code became a suffix. In 1958, the codes were adapted to conform with the ABC 123 numbering format. By 1980 the system became too complex to manage and the state discontinued the practice.
| County | Seat | 1935-1957 | 1958-1981 |
| King | Seattle | A | AAA-AZZ, OAA-OZZ, IAA-IZZ, UCA-UKZ |
| Pierce | Tacoma | B | BAA-BZZ, TBA-TFZ |
| Spokane | Spokane | C | CAA-CZZ, SLA-SMZ (excl CLx, CMx, CUx) |
| Snohomish | Everett | D | DAA-DVR (excl DSA-DUZ) |
| Yakima | Yakima | E | EAA-EKZ |
| Whatcom | Bellingham | F | FAA-FFP |
| Clark | Vancouver | G | GAA-GND, HHA-HHZ |
| Grays Harbor | Montesano | H | HAA-HDP |
| Kitsap | Port Orchard | I | KSA-KYZ |
| Thurston | Olympia | J | JAA-JEE |
| Chelan | Wenatchee | K | KAA-KDC |
| Lewis | Chehalis | L | LAA-LDG |
| Skagit | Mount Vernon | M | MAA-MDS |
| Cowlitz | Kelso | N | NAA-NFR |
| Walla Walla | Walla Walla | O | WWA-WYP |
| Whitman | Colfax | P | PAA-PCF |
| Clallam | Port Angeles | Q | CLA-CMZ, PDA-PDF |
| Benton | Prosser | R | RAA-REU |
| Kittitas | Ellensburg | S | SAA-SBH |
| Lincoln | Davenport | T | TAA-TAN |
| Okanogan | Okanogan | U | UAA-UBJ |
| Pacific | South Bend | V | VAA-VAW |
| Stevens | Colville | W | WAA-WBC |
| Mason | Shelton | X | XAA-XAZ |
| Jefferson | Port Townsend | Y | YAA-YAM |
| Klickitat | Goldendale | Z | ZAA-ZBD |
| Adams | Ritzville | AD | YYA-YYR |
| Asotin | Asotin | AN | ZYA-ZYL, ZZA-ZZZ |
| Columbia | Dayton | CO | CUA-CUG |
| Douglas | Waterville | DO | DSA-DST |
| Franklin | Pasco | FN | FNA-FSF |
| Ferry | Republic | FY | FYA-FYE |
| Garfield | Pomeroy | GA | GDA-GDF |
| Grant | Ephrata | GT | GTA-GVW |
| Island | Coupeville | IS | 1958: CVA-CVL 1963 up: ISA-ITH |
| Pend Oreille | Newport | PO | PEB-PES |
| San Juan | Friday Harbor | SJ | SJA-SJG |
| Skamania | Stevenson | SA | SKA-SKJ |
| Wahkiakum | Cathlamet | WA | WMA-WMF |
| Dept. of Licensing | JJ | JJA-JJZ, XXA-XXZ | |
| Dept. of Licensing (1939-1942) | LD | ||
