In React Native, how do I put a view on top of another view, with part of it lying outside the bounds of the view behind?

Add the following style to the "floating" view:

position: 'absolute'

You may also need to add a top and left value for positioning.

The above solutions were not working for me. I solved it by creating a View with the same background colour as the parent and added negative margin to move the image upwards.

<ScrollView style={{ backgroundColor: 'blue' }}>
  <View
    style={{
      width: '95%',
      paddingLeft: '5%',
      marginTop: 80,
      height: 800,
    }}>
    <View style={{ backgroundColor: 'white' }}>

      <Thumbnail square large source={{uri: uri}} style={{ marginTop: -30 }}/>
      <Text>Some Text</Text>
    </View>
  </View>
</ScrollView>

and I got the following result.

You can use zIndex for placing a view on top of another. It works like the CSS z-index property - components with a larger zIndex will render on top.

You can refer: Layout Props

Snippet:

    <ScrollView>
          <StatusBar backgroundColor="black" barStyle="light-content" />
          <Image style={styles.headerImage} source={{ uri: "http://www.artwallpaperhi.com/thumbnails/detail/20140814/cityscapes%20buildings%20hong%20kong_www.artwallpaperhi.com_18.jpg" }}>
            <View style={styles.back}>
              <TouchableOpacity>
                <Icons name="arrow-back" size={25} color="#ffffff" />
              </TouchableOpacity>
            </View>
            <Image style={styles.subHeaderImage} borderRadius={55} source={{ uri: "https://upload.wikimedia.org/wikipedia/commons/thumb/1/14/Albert_Einstein_1947.jpg/220px-Albert_Einstein_1947.jpg" }} />
          </Image>

const styles = StyleSheet.create({
    container: {
        flex: 1,
        backgroundColor: "white"
    },
    headerImage: {
        height: height(150),
        width: deviceWidth
    },
    subHeaderImage: {
        height: 110,
        width: 110,
        marginTop: height(35),
        marginLeft: width(25),
        borderColor: "white",
        borderWidth: 2,
        zIndex: 5
    },