We will discuss mixed state entanglement in random tensor networks, using the notion of negativity. Tensor networks are useful in many-body physics, as well as in holography, particularly in fixed-area states. We find the negativity spectrum, which is the analog of entanglement spectrum, in a general random tensor network with large bond dimensions, using a related flow network problem. We also study mixed states of a system coupled to gravity, such as a black hole with its Hawking radiation. We find island contributions in such systems, and show the Euclidean wormhole origin of these contributions in a Jackiw-Teitelboim gravity model.