Care was excluded from England's Six Nations plans following his arrest for drink driving in the early hours of New Year's Day, one of four recent brushes with the law.
The Harlequins scrum-half is currently on police bail after being arrested on suspicion of sexual assault, a claim he denies, but Lancaster has been assured the case will not impact on the tour.
The England coach has met with Care and is confident the 25-year-old now has a clear understanding of what is expected of him.
"In order to be an England player on the field you have to be able to lead yourself off the field. Danny needed to reassure me he could," Lancaster said.
"I met Danny. I am pleased with the progress he has made and the maturity he is now showing off the field.
"It is a great opportunity but equally he knows he is treading a very fine line, but I trust him enough to have the confidence to take him.
"He is a player in form and playing at a good club who are in a big (Premiership) semi-final this weekend.
"I spoke to the solicitor involved (in the case) and there will be nothing happening during the tour. There is no period where he will be brought back during the tour.
"I am very much with Conor (O'Shea, Harlequins director of rugby) on this one - we understand the context of what went on that particular evening and Danny's selection is the right one."
Saturday's semi-final will see Care, who has been in the best form of his career this season, go head-to-head with Northampton's Lee Dickson, the incumbent England scrum-half.
Care revealed this week he had not touched alcohol since the middle of March in a bid to get his England career back on track.
"It's been a while off the booze now. I don't have a drink problem - but I've made bad decisions when I've been drinking and now, hopefully, the trouble will go away," Care said.
"The common theme is that I was getting into trouble when I was out drinking and that's why I stopped."
James Haskell has also made a return to the England squad after opting out of the Six Nations to play for the Ricoh Black Rams in Japan and Otago Highlanders in New Zealand.
Haskell has been drafted into the squad, along with uncapped flankers Carl Fearns and Tom Johnson, after England lost Tom Wood and Tom Croft to injury and Calum Clark to suspension.
As the only squad player based outside the Aviva Premiership, Haskell will arrive late on tour but he offers the experience and physicality England will need against the Springboks.
Haskell was one of England's best players at the World Cup and he is returning to the Premiership next season with a move back to Wasps.
"I have never coached James. When I saw his performances in the World Cup and the Six Nations before then, he is clearly a very good player," Lancaster said.
"He is coming back to England and it is an opportunity, we have lost key men at blindside and I am looking forward to working for him."
Care and Haskell are not the only returnees. Harlequins wing Ugo Monye, Worcester prop Matt Mullan and Leicester centre Anthony Allen are all back involved in the senior England squad.
Lancaster has named 13 uncapped players, including the Wasps pair of wing Christian Wade and flanker Joe Launchbury, Saracens full-back Alex Goode and London Irish centre Jonathan Joseph.
Gilbert has released a new line of rugby cleats. The Gilbert Virtuo 8S is part of the exciting new product. Check it out.
The entire All Blacks apparel line has been updated for 2013/14. Check out the New Zealand All Blacks polo.
The Nike Tiempo is a solid rugby cleat and one of few styles still made from full-grain natural leather.
The Gilbert Blitz 8S rugby cleat is a great cleat at a great price of $69.99. Get a new pair of cleats today.
A cool looking all black rugby cleat with the high performance adidas is known for. Get in the Gear!
Wear the crest of the British and Irish Lions on your t-shirt. A great look for the summer.
The All Blacks Performance t-shirt is black with hints of blue from the training jersey. Very Cool.
The New Zealand All Blacks training jersey for 2013/14. Get in the Gear!
The USA Rugby Pro Alternate rugby jersey is perfect for any fan of the Eagles. Get yours to wear during the summer Test matches.
The NEW All Blacks 2013/14 jersey has arrived at World Rugby Shop. Dare to wear the colors of the All Blacks.