In a large saucepan, cover potatoes with 2 inches salted water. Bring to a simmer and cook until potatoes are just tender, 15 to 20 minutes. Drain thoroughly. In a large nonstick skillet, heat oil over medium-high; add potatoes and garlic and season with salt and pepper. Cook, mashing potatoes slightly, until golden brown in spots, about 5 minutes. Transfer to a serving dish and keep warm.
Add bacon to skillet. Cook until bacon is browned, about 3 minutes. Add green beans and cook, stirring occasionally, until bright green and crisp-tender, about 4 minutes. Add to potatoes along with lemon juice to taste. Season with salt and pepper and toss gently to combine.