Malloy hugged each of the parents and gave them a pen he used to sign the bill.
"We have come together," he said, "in a way that relatively few places in our nation have demonstrated an ability to do."
Since the Dec. 14 shooting in which 20 children and six educators were killed, some of their family members have become gun-control advocates, pressing for both tougher state and federal laws.
"This is a path I never thought my life would take. But working to save the lives of others is one way that I can honor Dylan's life," said Nicole Hockley, referring to her 6-year-old son who was killed at Sandy Hook. "We want Newtown to be known not for our tragedy, but for transformation."
Malloy said he has become friends with some of the parents and promised to keep working with them to enact further law changes that address gun violence.
Malloy and gun-control advocates said they hoped the new law, crafted by legislative leaders from both parties during several weeks of negotiations, coupled with President Obama's planned visit to the state Monday, would spur action in Washington.
In an interview on Fox News, NRA executive vice president Wayne LaPierre criticized the legislation.
"The problem with what Connecticut did," he said, "is the criminals, the drug dealers, the people that are going to do horror and terror, they aren't going to cooperate."
At the stroke of Malloy's pen on Thursday, the new law added more than 100 firearms to the state's assault weapons ban, effective immediately.
The new law also immediately bans the sale of magazines capable of holding more than 10 rounds of ammunition. People who purchased those guns and magazines before midnight Wednesday would be allowed to keep them, so long as they're registered with the state police before Jan. 1. Required background checks for private gun sales also take effect.
Other parts of the new law that take effect over the coming year include a ban on armor-piercing bullets, establishment of a deadly weapon offender registry, expansion of circumstances when a person's mental-health history disqualifies them from holding a gun permit, mandatory reporting of voluntary hospital commitments, doubled penalties for gun trafficking and other firearms violations, and $1 million to fund the statewide firearms-trafficking task force.