Combine the mashed strawberries, sugar, and lemon juice in a large saucepan.
Cook over medium heat, stirring constantly, until the sugar is dissolved.
Increase the heat to high and bring the mixture to a boil, stirring frequently.
Reduce the heat and simmer, stirring occasionally, until the mixture thickens, about 20-30 minutes.
Remove from heat and let cool. Transfer to jars and store in the refrigerator.