Giggs' first-half tap-in after Stephen Bywater had parried Cristiano Ronaldo's shot into his path, ended an anxious opening 40 minutes against relegation-threatened Derby and put United on the road to an eventual 4-1 win.
Typically, Giggs' landmark went largely unnoticed, with no announcement to the crowd, something Ferguson found strange given the 33-year-old's contribution to the United cause.
"I am surprised there was no announcement," said Ferguson.
"Maybe no-one knew but I certainly did. It is a fantastic achievement, absolutely marvellous.
"Not many players have scored 100 league goals but the service Ryan has given us, quite apart from the goals, is outstanding."
Although Derby worked hard, and gained some consolation when Steven Howard slid in to convert their first away goal of a league campaign that seems certain to end in relegation, they never seriously threatened once Giggs had struck.
Carlos Tevez added a second before half-time, then belted home his eighth goal of the season on the hour mark.
However, he was denied the opportunity to complete a hat-trick by Ronaldo, who rejected the South American's plea to take the penalty after the winger had earned a debatable decision from referee Chris Foy following Tyrone Mears' tackle.
"Carlos Tevez is making a great contribution this season," said Ferguson.
"Cristiano is our regular penalty-taker so I can understand why he wanted to take it but it would have been great for Carlos if he had scored a hat-trick."