Midfielder Wijnaldum, 24, signed for £14.5m from PSV, while £13m striker Mitrovic, 20, joined from Anderlecht.
Owner Mike Ashley has been heavily criticised for a lack of investment in player signings in recent seasons.
"It shows where we want to go. It's a statement of intent," the former England boss told BBC Newcastle.
"We've had to be patient and the first two through the door are the right age and the right experience."
|Newcastle United's record signings|
Mitrovic, 20, was confirmed by the Magpies on Tuesday, having netted 36 league goals in two seasons at Anderlecht.
"His record is good, he's a young talent and his profile is ideal," McClaren added.
"He's a player we've been following for quite a while.
"We looked at his record, looked at a few of his games and [chief scout] Graham Carr watched him a lot of times.
"Credit to Graham and [managing director] Lee Charnley for getting it over the line."
Newcastle finished last season 15th in the Premier League, only staying up on the final day by ending a 10-game winless run with victory against West Ham.