In a large saucepan, toss the strawberries with the sugar and let stand, stirring occasionally, until the sugar is mostly dissolved, about 1 hour.
Squeeze the lemon over the strawberries, add it to the saucepan and bring to a boil, stirring until the sugar is dissolved. Cook over moderately high heat, stirring, until the strawberries are just softened, 5 minutes. Add the mixed berries; cook over moderate heat until the liquid runs off the side of a spoon in thick, heavy drops, 20 to 25 minutes. Skim any scum that rises to the surface.
Discard the lemon; spoon the jam into three 1/2-pint jars, leaving about 1/4 inch of space at the top. Close the jars and let cool to room temperature. Store in the refrigerator for up to 3 months.