Mills and Clark, competing at only their second regatta since winning 470 silver at the London 2012 Olympics last summer, completed a British double in the class on the penultimate day in Spain.
With Luke Patience and Joe Glanfield securing the men's 470 crown a day early in Santander, Mills and Clark won their ten-boat double points medal race to seal the women's title.
Sophie Weguelin and Eilidh McIntyre finished just behind in second in that medal race for third overall at the regatta, also the test event for the 2014 ISAF Sailing World Championships at the same venue.
Patience and Glanfield were second in their own, albeit pressure free, medal race having secured the title a day early with Mills relieved that her and Clark are finally rediscovering their form.
"It's always nice to win an event, especially an international one," said Mills, who placed 12th with Clark at last month's World Championships in La Rochelle. "To be honest, Sas and I are just finding our rhythm again really.
"It's taking a bit longer than we'd have hoped it would take, but definitely the last part of this week we've really felt like we've made steps forward again in getting back towards where we were last summer.
"It's been a really positive event for us regardless of the outcome. We've got a good couple of months now to actually get some training in and really establish our equipment for next year and where we want to go with that.
"But we will also get our teamwork back up to where it was last summer. We're definitely on a good route for that now and we just need to keep it going over the winter."
Windsurfers Bryony Shaw and Nick Dempsey both advanced into the podium positions of the women's and men's RS:X respectively, the former second overall and the latter third,
Chloe Martin picked up another race win and a ninth to head into the Laser Radial medal race in bronze medal position with Hannah Snellgrove also qualifying in seventh.
Nick Thompson is assured of a podium finish in the Laser and will be joined in the medal race by fellow Brits Alex Mills Barton and Martin Evans while Ben Saxton and Hannah Diamond head into the Nacra 17 finale in equal first.
Rupert White and Nikki Boniface, Lucy Macgregor and Tim Carter and Pippa Wilson and John Gimson occupy third to fifth in the same class going in the medal race.
© Sportsbeat 2013