FlatList using 2 columns. I have an odd number of items to display. How do I get the last item to align left?

You can just add flex: 0.5 to item container:

 <FlatList
      columnWrapperStyle={{justifyContent:'space-between', }}
      data={[{key: 'a'}, {key: 'b'}, {key: 'c'}]}
      keyExtractor={item => item.itemId}
      horizontal={false}
      numColumns={2}
      renderItem={({ item, index }) => (
        <View style={{backgroundColor:'red', flex: 0.5, margin: 4}}>{/*Here*/}
          <Text>{index}</Text>
        </View>
      )}
    />

You need to change columnWrapperStyle

columnWrapperStyle={{justifyContent:'space-between'}}

you can also use alignItems:'flex-start'

Demo

let width = Dimensions.get('screen').width/2 - 8

return (
  <View style={{ flex: 1.0 }}>
    <FlatList
      columnWrapperStyle={{justifyContent:'space-between', }}
      data={[{key: 'a'}, {key: 'b'}, {key: 'c'}]}
      keyExtractor={item => item.itemId}
      horizontal={false}
      numColumns={2}
      renderItem={({ item, index }) => (
        <View style={{backgroundColor:'red', width:width, height:width, margin: 4}}>
          <Text>{index}</Text>
        </View>
      )}
    />
  </View>
);

FlatList Image