Place 9-inch square cake pan in freezer to chill.
Macerated Strawberries: In small bowl, stir strawberries with sugar; let stand until syrupy, about 30 minutes.
Ice Cream: Meanwhile, in blender, purée strawberries with sugar; strain through fine-mesh sieve into large bowl, pressing on solids with back of spoon and scraping bottom of sieve to extract pulp. Discard solids. Whisk in condensed milk and mascarpone.
In bowl, beat cream until stiff peaks form, about 3 minutes; fold into mascarpone mixture just until combined.
Scrape half of the mascarpone mixture into chilled pan; top with half of the Macerated Strawberries. Repeat with remaining mascarpone mixture and Macerated Strawberries; using small knife, swirl together to create marble effect. Freeze for at least 5 hours before serving.