The Swede was asked to take the reins temporarily after Unai Emery was sacked in November last year.
Ljungberg asked academy director Mertesacker to help him during the six-game stretch until Mikel Arteta was appointed, and the German was grateful for the experience.
"You know sometimes you close a chapter and think, 'first-team football, that's away from me right now, I have a new challenge in the academy', but ultimately it's the football club we are serving and whatever I am needed to do, I will step in," Mertesacker told Arsenal Player.
"When the change in management came and Freddie asked me to be his co-pilot, to support him, this is what the club needs me to do right now so I was really, really grateful for the opportunity to support and help Freddie with my energy and everything I could give to the players and to the club.
"It got extended to three weeks and it was kind of a whirlwind in terms of the intensity of first-team football. I almost forgot about [the intensity] so I am reflecting back now and thinking it was a fantastic time because it showed me how quickly it goes in football and how you need to stay ready for whatever opportunity and possibility comes for you in life.
"Stepping up for your football club, who give you a lot of trust, not only as a player, as a captain, and now as an academy manager, I take a lot of pride in those opportunities. I am just blown away by the trust in myself to lead the academy but as well stepping in where I can for the first team.
"It reiterated a lot of messages, being together with the first team then coming back to the academy and still being wanted, so it was a big whirlwind through December time. Our new baby was born during that time so you can almost say that every day felt like an hour.
"It was really rushed and now football, with the pandemic, it kind of slows down and everything I have missed in terms of taking care of the family, the players, and making sure I do the right thing, is put back now so naturally throughout the pandemic life slows down a little bit."