Lambertville is about 65 miles northeast of Philadelphia and 60 miles southwest of New York City. By car from Philadelphia, take I-95 north to New Hope on the Pennsylvania side and cross the bridge, or take US-202 north directly into Lambertville — about 75 minutes depending on traffic.
From New York, NJ Transit runs buses to Trenton, from where you would need a car or taxi north to Lambertville (about 20 miles). There is no direct train service to the town. The most practical option is to drive and park near the Delaware River — there is free on-street parking and a small lot near the towpath, though both fill up on busy weekend afternoons.
Coming from New Hope on the Pennsylvania side, walk across the pedestrian bridge — the most pleasant arrival possible.