Walker-Hebborn missed out on the required 200m backstroke time earlier this year but clocked 1:57.17, well inside the selection standard, to move ninth in the world rankings.
“I’m relieved. I wanted that so much,” said Walker-Hebborn.
“It is an absolutely brilliant feeling, I’m so happy.
“That race was for my mum, she’s an absolute star. She has been there through everything and she’s my biggest believer. So that was for her.”
Ieuan Lloyd, who has already booked a relay slot at the Games, won the 200m freestyle title in a new personal best of 1:47.55 - also inside the required A standard.
“It is an amazing feeling,” said Lloyd, after finishing just ahead of David Carry.
“I had to push out hard and on the third length I really went for it and made a bit of a break from the crowd.
“On the last length I was breathing towards David Carry and I could see him coming back but I was telling myself no, not this time and I managed to find the wall before he did.”
But there was disappointment for Antony James, who despite winning the 100m butterfly, finished one tenth of a second outside the qualifying time.