The Milwaukee and St. Louis flights will begin June 13 three days a week, at introductory fares as low as $59 each way to Milwaukee and $55 each way to St. Louis, if booked at FlyFrontier.com through 11:59 p.m. Sunday.
The Minneapolis flights will begin June 14 four days a week, with introductory fares starting at $75 one way, for travel through Aug. 10.
Seats are limited at these fares and certain flights or days of travel may be unavailable, the airline said.
Frontier is considering additional leisure destinations next winter and has talked with Mercer County officials about requirements to fly "a slightly longer route," including to Denver, Shurz said.
"Never before in its history has Trenton-Mercer Airport offered this number and variety of destinations, and we are thrilled with Frontier's plan to further expand its network," said Mercer County Executive Brian M. Hughes.
Frontier has chosen destinations, such as Minneapolis, where fares are high from Philadelphia and Newark, N.J., airports, Shurz said.
For example, a nonstop Philadelphia-Minneapolis flight, leaving April 14 and returning April 15, was $360 each way, or $720 round trip, on US Airways' website Wednesday.
A nonstop flight to Minneapolis from Newark Liberty International Airport was $832 on those dates, according to United Airlines' website.
Shurz told Brett Snyder, author of CrankyFlier.com, in November: "The Northeast never had low fares to the same extent" as other regions because Southwest Airlines, traditionally a low-fare carrier, "was never that big in the Northeast."
Frontier, which recently added a third 138-seat Airbus 319 aircraft to the Trenton schedule, will fly daily this summer to Atlanta; Raleigh-Durham, N.C.; and Chicago-Midway Airport; and five times a week to Detroit.
"We are very pleased with what we saw this winter for our leisure destinations and we continue to look at further opportunities to add leisure service," Shurz said.
"We are working with Mercer County to look at what the opportunities are . . . to take off in the Northeast direction, which would allow longer-range flights," Shurz said. "We believe that would allow Denver."
At the end of April, Frontier will have 57 flights a week from Trenton, including new flights to Cleveland, Cincinnati, Indianapolis, Nashville, and Charlotte, N.C.
Flights to St. Augustine, Fla., start May 2.
Frontier also flies nonstop from Trenton to Fort Lauderdale, Fort Myers, and Tampa, Fla., as well as Orlando.
Frontier's strategy has been to use a mix of smaller secondary airports, where landing fees are lower but which are close to large metropolitan areas, as well as to fly from some major airports.
Frontier, whose planes' tails have pictures of animals, often operates routes two or three times a week instead of daily. That requires fewer crews and usually means planes are full.