The agreement lays out a four-part plan of cooperation.
First, the agencies have agreed to coordinate efforts to have fast communication when needed.
Second, the agencies will provide each other with materials and operative help in the persecution of crimes and in the service of public safety when the issue transgresses the agencies' territories.
Third, the agencies will share information when necessary to apprehend suspects.
Fourth, the agencies will share in instruction to have more professional police forces on both sides of the border.
Calexico Mayor Victor Carrillo said the signing of the declaration was an important development.
He said the communities have a unique opportunity to work together to secure the safety, health and welfare of people on both sides of the border.
"We both serve as gateways to our countries," Carrillo said.
He added, "We might be separated at the fence, but we are not separated at the heart."
Tunson said Calexico residents will see a direct benefit from the formalizing of the relationship with Mexicali police.
He said the declaration sends a message that if a crime is committed, fleeing to Mexicali does not mean a suspect will escape authorities.
"A lot of people think they can leave the country," Tunson said. "That escaping will not happen."
The Calexico police force has about 100 staffers serving a city with a population of just under 30,000, although the number of people in the city can reach 80,000 on any given day throughout the year.
The Mexicali police force has 1,200 officers and 250 administrators serving a population of 800,000.
Tunson said the sharing of resources is one way to make sure residents on both sides of the border can have a sense of security when traveling through the communities.
As part of Thursday's signing Calexico City Councilman John Renison called for a moment of silence for Maria Cristina Ramos de Hermosillo, the wife of Mexicali's mayor and herself a well-known local politician and activist. She died of leukemia Wednesday.
Carrillo said Mrs. Hermosillo was "dynamic and socially conscious; a humanitarian of the highest order."
He added, "Her loss is deeply felt on both sides of the border."
Staff Writer Darren Simon can be reached at 337-4082.