Set back from the road on 2.7 beautifully landscaped acres, 451 Mansfield Avenue offers the privacy, grandeur, and distinctive character of a true country estate. A sweeping circular drive introduces a grand residence distinguished by its striking stone facade and timeless architectural detailing, a substantial two-story guest house, and a magnificent outdoor setting centered around a 24' x 48' infinity-edge pool. Extensive custom stonework, elegant terraces, and lush grounds enhance the property's distinctive estate-like character. Extensively renovated in 2026, the residence features all- new mechanical systems, new windows, and thoughtfully redesigned interiors. Blending timeless architectural elegance with sophisticated contemporary luxury, the home offers approximately 9, 000 square feet of finished living space, including a light-filled 1, 600-square-foot walk-out lower level. Inside, a graceful, curved staircase rises from the herringbone-floored foyer, introducing beautifully proportioned rooms, distinctive millwork, and refined architectural detailing. Formal living and dining rooms, each with a custom marble fireplace, provide gracious entertaining spaces, while a richly paneled library with coffered ceiling and fireplace offers a distinguished private study., At the heart of the home is a stunning chef's kitchen featuring custom cabinetry, warm wood accents, a dramatic 6' x 10' Calacatta Black marble island, and full-height marble backsplash. A French La Cornue range, Wolf ovens and coffee system, Miele dishwashers, integrated refrigeration, and wine column complete the space. The kitchen flows into a sunlit breakfast area and fireside family room, complemented by a walk-in pantry and mudroom. The luxurious primary suite features its own fireplace, a 14' x 14' dressing room with center island, an additional walk-in closet, and a spa-inspired bath finished in Taj Mahal quartzite, with a copper soaking tub and arched tiled shower. The main residence offers six bedrooms, seven full and two half baths, a bonus room, and second-floor laundry. An additional finished upper level, connected by an open staircase, provides two versatile living spaces and a full bathroom. The beautifully finished walk-out lower level extends the home's living and entertaining spaces, with abundant natural light and French doors opening directly to the pool terraces. Highlights include a fireside lounge, billiards room, custom Sienna Gold quartz bar, and a versatile glass-enclosed fitness or recreation studio with built-in sauna. The outdoor setting is among the property's most compelling features. Expansive stone terraces, illuminated stairways, an outdoor kitchen, and fire pit surround the infinity-edge pool, creating the atmosphere of a private luxury resort. The substantial guest house, with its own bedroom, two full baths, and covered loggia featuring a stone fireplace, adds another dimension to the estate, ideal for guests and poolside entertaining. With a four-car garage, exceptional privacy, and convenient access to downtown Darien, Metro-North, and area beaches, 451 Mansfield Avenue offers a rare combination of architectural presence, custom craftsmanship, and resort-style living in one of Fairfield County's most desirable communities.