The one-time England lock will take charge of the Clifton Lane club from Monday, November 26, and has signed a two-year deal to succeed Andre Bester.
The 39-year-old Londoner turned down the chance to take charge of Rotherham in the summer of 2010 to concentrate on his teaching career.
He combined that with roles at Barking and then as forwards coach of London Scottish, but kept in regular contact with the Titans heirarchy.
Codling will watch Rotherham’s next game against Yorkshire rivals Leeds next Saturday before taking training for the first time 48 hours later.
Building on the steady start Rotherham have made is his aim for the remainder of the season, with the question of whether he can ultimately get the club promoted to the Premiership during his tenure an ambition he is not ready to broadcast.
“As a player and a coach I have always been incredibly ambitious. I’ve always wanted to be the best I can be,” said Codling, who played for Harlequins, Saracens and Montpellier.
“First and foremost this is about developing the players technically and tactically.
“There’s a great group of guys here and and they work very hard and the task is to develop these players further.
“When you’re in a league you’re there to win it. But you have to be realistic and consider what team has come down from the Premiership, like this season for instance, with Newcastle coming down and the strength of resources and coaching and players that they have.
“It’s always dangerous to make bold predictions. Ultimately the goal is to win as many games as I can.
“The one thing I will always be is realistic, and I will balance that realism with ambition.
“Rotherham have started well in the British and Irish Cup and are competitive in the league and the immediate aim is to keep pushing on in both areas.”
On a personal level, the former second-row forward who was capped by Sir Clive Woodward against Argentina in 2002, is delighted to have been given the chance to lead one of the proudest names in northern rugby.
“I’m very much looking forward to the challenge,” said Codling, who has experience of coaching with England and the RFU.
“I’ve spoken to Martin (Jenkinson) and Nick (Cragg) on numerous occasions over the last couple of years.
“It’s a traditional club, a homely club, one that’s very positive and very ambitious. It is a club with a great tradition and passionate supporters. I share their passion for the club and this great game.
“Up until now I’ve juggled rugby with teaching but I’m ready to take the plunge.”
Titans chairman Cragg said: “We were delighted by the quality of candidate and the general level of interest shown in the post.
“We have undertaken a very thorough process.”
Director of rugby Martin Jenkinson added: “We believe Alex is the outstanding choice to be our next head coach.
“His record as a player and a coach is of the highest order and his overall track record is excellent.
“Alex is highly respected within the game and we believe he has the right values to build a great Rotherham team and to take the club forward on the field of play.”