The trio made their first appearances in a Liverpool shirt this summer, with Wijnaldum signing in a £25 million deal on Friday, while Origi and Mignolet returned from holiday.
First-team development coach Pepijn Lijnders and fitness coach Andreas Kornmayer oversaw an intense session in the California heat.
Wijnaldum and Origi are expected to play a big part in Klopp’s long-term plans in attack, but one player conspicuous in his absence was Belgian striker Christian Benteke.
The 25-year-old is poised to leave the club this summer, with Crystal Palace the most likely destination.
Klopp’s England internationals have also returned to action, following their exploits at this summer’s European Championship.
The likes of Roberto Firmino, Alberto Moreno, Marko Grujic, Philippe Coutinho, Danny Ings, Sadio Mane, Loris Karius, Lazar Markovic, Cameron Brannagan, Andre Wisdom, Dejan Lovren, Ragnar Klavan, Kevin Stewart, Trent Alexander-Arnold and Ovie Ejaria also featured.