I’d start by checking what the furniture is made of and how it’s finished, because the safest method depends on that. For most powder-coated metal, sealed wood, plastic, and resin wicker, I use the gentlest approach first: mix a few drops of mild dish soap in warm water, wipe with a soft microfiber cloth or soft brush, then rinse and dry thoroughly. Don’t scrub hard or use steel wool, scouring pads, or a pressure washer—those can dull paint, scratch clear coats, and force water into joints.
For actual mold on urban outdoor furniture, I like a diluted white vinegar solution—about 1 part white vinegar to 1 part water—for many finishes, but I always test a hidden spot first. Spray or wipe it on, let it sit for 10 to 15 minutes, then wipe away and rinse. If the mold is stubborn, an oxygen-based cleaner can be gentler than chlorine bleach on some finishes, but check the manufacturer’s care guide first. Avoid bleach on colored fabric, unsealed wood, and certain metals; it can strip color and damage the surface. For metal, dry immediately to prevent rust. For wood, use a cleaner made for sealed outdoor wood and reapply the appropriate sealer if needed.
In cities, grime and pollution can hold moisture, so clean more often than you think—especially after rainy or humid spells. I also remove cushions, brush off loose dirt, and let everything air-dry in the sun before covering it. If you cover furniture, use a breathable cover or leave a little ventilation; trapping dampness invites mold back. A quick monthly wipe-down and keeping furniture out of constant damp shade will do a lot to protect the finish.