Both claimed titles on the opening day of competition in Sheffield.
Williams broke the British age group record on his way to victory in the 13/14 years’ 100m breaststroke, while Commonwealth Games swimmer Stephens smashed her personal best to win the women’s 400m individual medley.
Darts’ Williams caused a stir in the morning session at Ponds Forge by knocking nearly four tenths of a second off Craig Benson’s British record for 14 year-olds which had stood since March 2009.
Williams, who had broken the record for 13 year-olds at the Winter Championships, clocked 1:05.06 in the heats before swimming 1:05.37 to win by more than a second in the final.
Leander’s Stephens then put on her own show in the senior women’s 400m IM.
The butterfly specialist knocked more than eight seconds off her previous best time with a 4:48.77 minute swim.
She finished nearly three seconds ahead of second-placed Orla Adams of Aberdeen.
“That was a really nice surprise,” said Stephens afterwards.
“I am really happy with that for day one. To get a positive on the first day builds the momentum.”
Stephens was cheered on to victory by some vocal support from her Leander team-mates.
“It’s great to have that presence on poolside. It just helps lift everyone. I’m really proud to be part of it.”
Stephens’ Leander team-mates Honey Osrin and Lucia Grant came agonisingly close to medalling on the opening night of competition in Sheffield. Both ended up having to settle for fourth-placed finishes, as did Mount Kelly’s Sam Dailley.
Osrin took fourth in the 15 year-olds’ 200m freestyle with a 2:04.13 swim, while Grant finished in the same position in the same age category of the 50m breaststroke with a time of 32.80 seconds.
Dailley, meanwhile, was part of a Mount Kelly trio who finished fourth, fifth and sixth in the 17/18 years’ 50m backstroke. Dailley clocked 26.65 seconds, Harry Nobel was fifth in 26.75 and Alex Mason sixth in 26.77.
Fellow Mount Kelly College swimmer Oliver Taverner came sixth in the 17/18 years’ 100m breaststroke in 1:04.66. Leander’s Liam Davis was eighth in 1:05.78.
There was also an eighth place finish for Leander’s Adam Chillingworth in the final of the senior men’s 100m breaststroke with a 1:02.58 swim.
Other finalists on the first day were Mount Kelly’s Mason Penny and Benjamin Grainger and Dartmoor Darts’ Hollie Steer.
Penny was seventh in the 17/18 years’ 50m breaststroke, while Steer was ninth in the girls’ 16 years’ one-length breaststroke. Grainger was eighth in the MC 400m freestyle in 4:41.74.
The championships continue in Sheffield until Sunday.