Toner has signed a two-year deal with the IRFU which will see him remain at the RDS Arena until at least June 2017.
The lock made his senior debut for the province in 2006 and has clocked up 157 appearances since - scoring three tries in the process.
Six of those appearances have come in the GUINNESS PRO12 this season and Cullen, who turned out for Leinster over 200 times during his own playing career, admits Toner is a crucial cog at the club.
"It is great news," Cullen told the Irish Times. "Dev is such a key part of what we do here.
"I've worked very closely with Dev over the past number of years and he's grown into himself as a character.
"Sometimes it takes a bit longer for guys to mature into their bodies. He has a lot of experience now of playing games and he's a starter for Ireland.
"A few guys, I know, have been out injured, but he's established himself at that highest level.
"So yeah he's a key guy for us and it's a great announcement for Leinster that he wants to stay here.
"It's a good statement that he wants to stay here and he sees himself as part of Leinster for the next couple of years."
In addition to being a regular in Matt O'Connor's starting XV, Toner has also established himself as a fixture in the Ireland set-up over the past two years.
And the 28-year-old acknowledges that he wouldn't be the player he is today without the support from Leinster.
"It is great to have signed on for another two years with Leinster," said Toner. "I have been with the province for almost a decade and during that time the club have invested a lot in my development.
"They have given me the platform to reach my potential and achieve success at both provincial and national level."
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