US Post Office is located at 191 Riverside Ave, Newport Beach, CA. This location is in the Mariners Mile neighborhood. This business specializes in Post Offices. US Post Office is open Mon, 8:05am-5pm; Tue, 8:05am-5pm; Wed, 8:05am-5pm; Thu, 8:05am-5pm; Fri, 8:05am-5pm; Sat, Closed; Sun, Closed and accepts American Express, Discover, Mastercard and Visa.