In an all-Aboyne final against 18-year-old Kimberley Beveride, Shannon triumphed by 2 and 1.
That honour was held by Elaine Farquharson (Deeside) who was 15 when she won the championship - at Aboyne - in 1983.
Rain ruined this year's final as a spectacle and, indeed, forced a suspension of play for over half an hour at the eight when the course in that area was beginning to show signs of flooding.
Beveridge had taken a two-hole lead by winning the third and second but her advantage was wiped out when McWilliam won the seventh and ninth with pars to be all square at the turn.
McWilliam, last year's Scottish Under-14 girls champion, kept her momentum going by winning the short 10th with a 2 to lead for the first time.
A par put her two up at the 12th.
Beveridge kept her hopes alive by winning the 13th with a par but, after the next three holes were halved, McWilliam clinched her historic victory with a birdie - her third of the match - at the 17th for a 2 and 1 win.
Underlining the youthful keynote of the championship, 14-year-old Jasmine Mackintosh (Hazlehead) won the Handicap championship. Playing off seven, she beat Lynne Terry, an eight-handicapper at Cruden Bay, by 4 and 3 in the final.
ABERDEENSHIRE WOMEN'S COUNTY CHAMPIONSHIP
S Mathieson (Murcar Links) bt L Watson (Deeside) 1 hole; S Wood (Aberdeen Ladies) bt M Stewqart (Murcar Links) 4 and 3; D Pocock (Murcar Ladies) bt J Whitehead (Kemnay) 1 hole; K Beveridge (Aboyne) bt K Stalker (Cruden Bay) 4 and 3.
L Coleman (Peterhead) bt M Johnstone (Murcar Links) 2 and 1; S McWilliam (Aboyne) bt T Seivwright (Murcar Links) 5 and 4; C Prouose (Hazlehead) bt S Robertson (Fraserburgh) 6 and 4; S Leslie (Aberdeen Ladies) bt F Hay (Deeside) 2 holes.
Wood bt Mathieson 4 and 3, Beveridge bt Pocock 2 and 1, McWilliam bt Coleman 6 and 4, Leslie bt Prouse 5 and 3.
Beveridge bt Wood 1 hole, McWilliam bt Leslie 2 holes.
McWilliam bt Beveridge 2 and 1
M Adam (Hazlehead) (16) bt L Watson (Deeside) (13) 3 and 2; L Terry (Cruden Bay) (8) bt E Whyte (Aberdeen Ladies) (7) 7 and 5; J Mackintosh (Hazlehead) (7) t K Guthrie (Inverurie) (7) 2 and 1; K Watt (Aberdeen Ladies) (8) bt L Park (Meldrum House) (12) 2 and 1.
Terry beat Adam 1 hole, Mackintosh bt Watt 6 and 5.
Mackintosh bt Terry 4 and 3.
CREDIT CAL CARSON GOLF AGENCY