Yes, technically you can leave cushions on urban outdoor furniture overnight, but I wouldn’t recommend making it a habit. In a city, your furniture has to deal with more than just rain. Overnight there can be dew, sudden showers, wind-blown dust, pollution, bird droppings, tree sap, and even curious pets or pests. Cushions left out can also become damp and start to smell musty or develop mold, especially if they sit in shade or against a wall where air cannot move.
If the cushions are outdoor-rated and quick-drying, one dry, calm night is probably not going to ruin them. But if rain is forecast, or if the cushions are thick, cotton-filled, or not really meant for weather, bring them inside. In shared or street-facing spaces, theft and vandalism are real concerns too. Even on a balcony or rooftop, wind can flip lightweight cushions over the railing or send them into a dirty corner.
If you really want to leave them out, use waterproof outdoor cushions or add a fitted waterproof cover. Secure everything with straps or clips, and put the cushions on a slight angle so water runs off instead of pooling. Avoid leaving them out after rain; let them dry fully before you stack them or put them into outdoor cushion storage. Also shake them off and wipe down hard surfaces regularly, because city dust and grime build up fast.
For city patio furniture, the safest routine is simple: bring cushions in overnight, or keep them in a weatherproof storage box nearby. If you must leave them on urban outdoor furniture, check the weather, cover them properly, and secure them. That small habit will keep them cleaner, drier, and usable for much longer.