Slater picked up her fifth individual gold of the event and also set a European record after coming home first in the women's S8 50 metres freestyle in 30.44 seconds, almost a second quicker than the previous mark.
The world record holder said: "It was a really good swim. I love the 50m freestyle - it's a great sprint event and I love my sprints. I've always loved doing the 50m and it's one I'll hopefully work on a lot more."
Slater then picked up a team gold after linking up with Steph Millward, Claire Cashmore and Amy Marren to win the women's 4x100m medley relay.
Mullen, meanwhile, also set a new European record of 37.56sec as he won the men's S5 50m backstroke for his fourth gold medal of the championships.
The Glaswegian said: "It's been a great week. I don't really think it could have gone much better for me. I have a 100 % PB record, four gold medals and two silvers - I don't think I could have asked for any more."
Double Paralympic champion Ellie Simmonds boasts three gold medals after adding the SB6 100m breaststroke title to her collection, while Marren won her sixth individual medal with silver in the S9 100m butterfly.
Hannah Russell also returns from Holland with six medals after winning silver in the S12 400m freestyle, while Jack Thomas took silver in the SM14 200m individual medley with Craig Harris taking bronze.
The men's 4x100m medley relay team of Ollie Hynd, Jack Bridge, James Hollis and Matt Walker also claimed silver, while Jessica-Jane Applegate earned a bronze in the SM14 200m individual medley.
Those successes helped the GB team finish with 73 medals - including 30 golds - to exceed their target for the championships.