Alderweireld has joined from Atletico Madrid on a season-long loan, and was swiftly joined by Mane from Red Bull Salzburg on a four-year deal.
And Southampton manager Ronald Koeman insisted Alderweireld is a player he has monitored closely for many years.
"I am delighted that we have been able to bring Toby to the club," said the Dutchman.
"I know him very well because he played for some years for Ajax in Holland and he then made a transfer to Atletico Madrid, where he hasn't been playing an awful lot, which is why he was looking for somewhere else to play.
"He is a strong defender, and is also very good at building up the play when we play out from the back.
"He played in the top league in Spain and against top teams in Europe, so he has a lot of confidence and experience, and he has a big future because he is still young.
"We needed competition in our squad, and with Toby here we now have a lot of possibilities and quality."
Alderweireld made 12 appearances last season as Atletico won La Liga and is thrilled at the prospect of potentially getting more first-team opportunities at Southampton.
He said: "I had a good talk with the coach.
"First of all, it was very important for me to play this season and the coach gave me a good solution for that.
"The trainer likes how I play and really wanted me to come here, which is what convinced me to come."
Senegal international Mane, meanwhile, is being tipped to be an exciting acquisition.
Koeman said: "He can play in different positions in the attack – he can play on the left or right side and also as a number nine.
"It is incredible how many goals he has scored from his position, and I hope that he will do the same for us."